This invention relates generally to exposure apparatuses, and more particularly to an exposure method and apparatus used to expose an object to be exposed, such as a single crystal substrate for a semiconductor wafer, and a glass substrate for a liquid crystal display (LCD), a device fabricating method using the exposed object, and a device fabricated from the exposed object. The exposure method and apparatus of the instant invention are applicable to the fabrication of various types of devices, for example, semiconductor chips such as ICs and LSIs, display devices such as liquid crystal panels, detecting devices such as magnetic heads, and imaging devices like CCDs.
The conventional photolithography for fabricating devices, such as ICs, LSIs, and liquid crystal panels, has utilized projection exposure methods and apparatuses. Such methods and apparatuses use a projection optical system to project or transfer a circuit pattern on a photo-mask or reticle (called xe2x80x9ca maskxe2x80x9d hereinafter) onto a photoresist-applied, photosensitive substrate, such as a silicon wafer and a glass plate (called a xe2x80x9cwaferxe2x80x9d hereinafter), thereby exposing the substrate with the circuit pattern.
The higher integration of these devices accordingly requires a smaller pattern to be transferred to a chip area on a wafer, that is, the higher resolution, as well as a larger area for each chip area on the wafer. Therefore, the projection exposure method and apparatus for taking a lead in the fine wafer processing technology are also required to improve the resolution and exposure area so that an image with a size (or line width) of 0.5 xcexcm or less can be formed in a wider area.
A typical schematic of a conventional projection exposure apparatus is shown in FIG. 13. In FIG. 13, 191 denotes an excimer laser as a light source for far ultraviolet exposure, 192 an illumination optical system, 193 an illumination beam, 194 a mask, 195 an object-side exposure beam emitting from the mask 194 and incident upon the optical system 196, 196 a demagnification projection optical system, 197 an image-side exposure beam emitting from the optical system 196 and incident upon the wafer 198 which is a photosensitive substrate, and 199 a substrate stage that holds the photosensitive substrate.
The laser beam emitted from the excimer laser 191 is directed to the illumination optical system 192 by a directing optical system, and then turned by the illumination optical system 192 into the illumination beam 193 with a specified light intensity distribution, a luminous intensity distribution, and an open angle (number of apertures NA). The illumination beam 193, in turn, illuminates the mask 194. The mask 194 forms on its quartz substrate a chromium pattern, which is a reciprocal times of projection optical system 196""s projection power (for example, twice, four times or five times) as large as the minute pattern formed on the wafer 198. The illumination beam 193 is diffracted after transmitting through the minute pattern on the mask 194, and turned into the object-side exposure beam 195. The projection optical system 196 converts the object-side exposure beam 195 into the image-side exposure beam 197 for forming an image representative of mask 194""s minute pattern on the wafer 198 with the above projection power and sufficiently small aberration. The image-side exposure beam 197 converges, as shown in the enlarged part at the bottom of FIG. 13, onto the wafer 198 with a specified NA (=sin xcex8), creating an image of the minute pattern on the wafer 198. In order to form a minute pattern sequentially on multiple different areas (or shot areas each of which will becomes one or more chips) on the wafer 198, the substrate stage 199 stepwise moves along the image plane of the projection optical system and shifts a position of the wafer 198 relative to the projection optical system 196.
However, it is difficult for the projection exposure apparatuses, which use currently widespread excimer lasers as a light source, to form a pattern of 0.10 xcexcm or less.
The projection optical system 196 has its limits in resolution based on the trade-off between the optical resolution R dependent on the wavelength of an exposure beam (called an xe2x80x9cexposure wavelengthxe2x80x9d hereinafter) and a depth of focus (xe2x80x9cDOFxe2x80x9d). R and DOF in the projection exposure apparatus are given as following Rayleigh""s formulas (1) and (2):
R=k1(xcex/NA)xe2x80x83xe2x80x83(1) 
DOF=k2(xcex/NA)xe2x80x83xe2x80x83(2) 
xcex is an exposure wavelength, NA is the number of apertures at the image side of the projection optical system 196, and values of k1 and k2 usually fall between about 0.5-0.7, at most about 0.4 even for a resolution enhancement like a phase shift. These formulas indicates that the superior resolution with a smaller value of R is obtainable from the higher NA or the increased number of apertures NA, but the projection optical system 196 needs relatively large DOF in the actual exposure and the NA can be increased to only some extent. As a result, it is understood that a shorter wavelength or reduced exposure wavelength xcex is needed for the higher resolution.
Nevertheless, the shortened wavelength would possibly raise a critical problem in that there would be no glass materials available for lenses in the projection optical system 196. Transmittances of most glass materials are close to 0 in the far ultraviolet range. Even synthetic quartz, which is fabricated by a special fabricating method, as a glass material for an exposure apparatus (with the exposure wavelength of 248 nm), drops its transmittance drastically for the exposure wavelength of 193 nm or shorter. It seems very difficult to develop practical glass materials as have sufficiently high transmittance for an exposure wavelength of 150 nm or less, which allows for a minute pattern transfer of 0.10 xcexcm or less. Further, glass materials for use in the far ultraviolet range need to satisfy such certain conditions in view of durability, uniform refractive index, optical distortion, and manufacturability, in addition to the transmittance, that practical glass materials for the exposure wavelength of 150 nm or shorter become harder to be available.
The conventional projection exposure method and apparatus thus need to use the shortened exposure wavelength of about 150 nm or below to form a pattern of 0.10 xcexcm or less on the wafer 198, but cannot do that since no practical glass material is available in this wavelength range.
In the meantime, a fine processing apparatus configured as a scanning near field optical microscope (referred to as an xe2x80x9cSNOMxe2x80x9d hereinafter) has recently been proposed to provide optical fine processing of a size of 0.1 xcexcm or less. This apparatus utilizes evanescent or near-field light oozed out of a fine aperture of, for example, 100 nm or less to locally expose the resist beyond the limit of a light wavelength. However, such a SNOM lithographic apparatus disadvantageously has a poor throughput since it is adapted to use one or more processing probes for the fine process as if drawing a picture with a single stroke of brush.
As in Japanese Laid-Open Patent Application No. 8-179493, one proposed solution for this problem provides a photo mask with a prism, leads light to the prism at an angle of incidence producing the total reflection, and utilizes the near field light oozed out of the total-reflection surface to transfer the entire photo mask pattern onto the resist at one time.
It is requisite for the batch exposure apparatus using the prism and near field light, as disclosed in the above reference, to set a distance between the prism/mask and the resist surface to 100 nm or less. However, in reality, it is difficult to set the distance between them to 100 nm or less throughout the entire prism/mask surface, due to the limitative surface precision and flatness of the prism/mask and the substrate. Any slight tilt in positioning the prism/mask relative to the substrate would make the distance setting difficult.
Such a non-uniform interval disadvantageously results in a non-uniformly exposed pattern, and partially crushed resist by the prism/mask.
One proposed solution for these problems provides an exposure method and exposure apparatus using evanescent light with an elastic mask having a minute aperture pattern with an aperture width of 100 nm or less on its first surface. The mask is made of elastic material and elastically transformable in the normal line direction on the mask surface. Thus, the apparatus and method exposes and transfers the minute aperture pattern onto an object to be exposed, which is arranged opposite to the front surface of the mask (Japanese Laid-Open Patent Application No. 11-145051).
Nevertheless, such exposure using a mask and near field light is still disadvantageous to practical applications in various difficulties, for example, in manufacturing a necessary minute mask of an equal magnification, and in an alignment between the near-field mask and the wafer.
On the other hand, there has been proposed a resist exposing method, which includes the steps of applying onto a resist a thin film which augments optical transmittance as the intensity of an incident beam increases, irradiating an optical spot to the thin film so as to increase the optical transmittance of the thin film locally, shaping the optical-transmittance increased part in the thin film into a desired pattern by relatively scanning the optical spot and the resist, and exposing the resist by way of the optical-transmittance increased part in the thin film (Japanese Laid-Open Patent Application No. 9-7935). This method employs an opaque thin film for the thin film having a given fusing or sublimation point, and increases the optical transmittance in the thin film by locally heating the thin film with the optical spot until the temperature exceeds the fusing or sublimation point. The thin film scanned by a laser scanning exposure apparatus heats up in a portion irradiated by the laser spot. The optical intensity distribution of the laser spot is generally in the form of a Gaussian distribution, and the temperature distribution in the thin film is approximately in the form of a Gaussian distribution accordingly. Therefore, the adjustment of the laser power would be able to raise the temperature in an area w only, which is far narrower than a laser spot xcfx86, higher than the fusing point. As the thin film in this area w melts down past its fusing point and the melted part allows a beam to pass, a linear scan of the laser spot, for example, would create a straight-line light transmitting part with a width of w, which is far thinner than the laser spot size xcfx86, on the opaque thin film. The light transmitting part thus formed on the opaque thin film may narrow its width w. In addition, when multiple straight-line light transmitting parts are formed at regular intervals, the cycle may be much smaller than the diffractive limit determined by the optical cutoff frequency. The resist is exposed by near-field light passing through the thus created minute aperture.
However, the exposure method disclosed in Japanese Laid-Open Patent Application No. 9-7935 could not stably derive its effect because the method directly applies the optical-transmittance increasing thin film onto the resist The uniform film formation has also been difficult since the conditional formation of the film onto the resist film relies upon resist""s characteristics. Moreover, it is arduous to form a film for each of thousands of objects to be exposed. Further, the resist process should burdensomely strip this thin film off from the resist after exposure. As a consequence, the exposure method described in this reference has a poor throughput disadvantageously. Due to the fine uneven surfaces of the resist as well as the underlying substrate, it is also difficult to laminate or form a thin film uniformly on the resist, whereby the exposure method in this reference disadvantageously has the poor overlay accuracy for exposure.
Accordingly, it is a general object of the present invention to provide a novel and useful exposure method and apparatus, device fabrication method, and device, in which the above disadvantages are eliminated.
More specifically, it is an exemplary object of the present invention to provide an exposure method and apparatus having an excellent throughput, overlay accuracy and resolution and device fabricating method, as well as a device.
An exposure method according to one aspect of this invention includes the steps of arranging an object to be exposed and a transparent plate that includes a thin film, within such a range that near field light from the thin film may operate on the object, the thin film having a light transmittance that changes according to an intensity of light of incidence, and exposing the object with near field light generated by projecting a pattern on a mask, onto the thin film of the transparent plate. An exposure apparatus according to another aspect of this invention includes a transparent plate, arranged within such a range that the near field light from the transparent plate may operate on an object to be exposed, said transparent plate including a thin film whose light transmittance changes according to an intensity of light of incidence, and a projection optical system for projecting a pattern on a mask onto the thin film of the transparent plate and exposing the object with near field light generated by the projection. Such an exposure method and apparatus utilize the near field light for exposure, create a thin film in the transparent plate as a member independent of the object, and use the projection optical system to project a pattern on a mask onto the thin film.
The thin film may be an optically or thermally active layer that reversibly generates a minute aperture using light or heat. It is therefore preferable that it is composed of a phase changing material or a material having a high tertiary nonlinear effect, preferably, antimony or an alloy of antimony as its main ingredient. It is desirable to additionally provide stabilizing layers that stabilize fluctuations in the light transmittance of the thin film and protect the thin film. A range in which the near field light operates on the object may be set, for example, as a separation between the thin film and the object that is from zero to a wavelength of the near field light or less. The thin film adapted such that it wholly covers the object would make it easy for the thin film and the object to contact to and exfoliate from each other. The thin film may be so elastically transformable that it wholly or partly contacts to and exfoliates from the object. According to Pascal""s principle, it is a good practice to use uniform pressure to transform the thin film such that it may contact to and exfoliate from the object.
It is preferable that such an exposure method and apparatus may perform various corrections to realize exposure of high accuracy. Such corrections may include the focus and tilt corrections for the object which optically detect the exposing position (focal position) of the object and then perform corrections based on the detected focal position, alignment of the object using an alignment mark provided on the object as well as corrections of an alignment aberration, corrections of the projection system""s aberration produced due to a thickness of the transparent plate, and the like. The corrections of the projection system""s aberration may be performed, for example, by changing a separation between the lenses in the projection optical system.
The device fabricating method according to still another aspect of the present invention includes the steps for utilizing the above exposure apparatus to project and expose the object, and performing a predetermined process on the object projected and exposed. The claim for the device fabricating method that expressly declares an operation similar to that of the above exposure apparatus continues to have its effect on a device as its intermediate and final product. Such devices include semiconductor chips such as LSI and VLSI, CCDs, LCD, magnetic sensors, and thin film magnetic heads.
Other objects and more characteristics of the present invention will be made clear by the preferred embodiments described referencing accompanying drawings as follows.